Cellulose acetate is an organic ester of cellulose. It remains as the largest and most important commercial cellulose derivative because of its wide variety of uses such as in textile fibers, tobacco filter tips, plastics, films and paints.
In a conventional preparation of cellulose acetate (as described, for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, vol. 5, pages 100 to 105, (1979), John Wiley & Sons, Inc.), the first step is to pretreat a cellulosic raw material such as wood pulp or cotton linter with a suitable amount of acetic acid.
Then, the resulting mixture is immersed in a cooled acetylating mixed solution to form primary cellulose acetate (completely esterified cellulose). In this step, the acetylating mixed solution comprises, for example, acetic acid as a solvent, sulfuric acid as a acetylation catalyst and acetic anhydride as an acetylating agent. The acetic anhydride is generally used in excess to the amount stoichiometrically equivalent to the sum of cellulose and water present.
After the acetylation step, an aqueous solution of a neutralizing agent, e.g. a carbonate, acetate, hydroxide or oxide of calcium, magnesium, iron, aluminum or zinc (preferably magnesium acetate), is added to hydrolyze the remaining acetic anhydride and to partly neutralize the acetylation catalyst.
In the next step, the ripening step, the resulting primary cellulose acetate is hydrolyzed by maintaining the ripening solution at 50.degree.-90.degree. C. while a small amount of acetylation catalyst (e.g., sulfuric acid) is still present, to transform the same to secondary cellulose acetate having a prescribed substitution degree of acetyl group and degree of polymerization. When the secondary cellulose acetate is obtained, the catalyst remaining in the ripening solution is generally completely neutralized with a neutralizing agent as exemplified above, though this additional neutralization may be omitted.
The ripened solution is then poured into water or dilute acetic acid or water or dilute acetic acid is poured into the ripened solution to separate the secondary cellulose acetate.
The secondary cellulose acetate is then washed and subjected to a stabilizing treatment to yield the desired final product.
Articles such as fibers, films or plastics made of cellulose acetate prepared by the above process are tinted pale yellow. This phenomenon is particularly remarkable with cellulose acetate produced from wood pulp as the raw material. For the commercial production of fibers, films or plastics, therefore, an after-treatment of treating or adding a bleaching agent, white pigment, fluorescent whitening agent or antioxidant has been applied before or at processing (e.g., spinning, molding) to reduce yellowness or improve whiteness. Such after-treatment, of course, is not completely effective and has only a limited effect.
The supply of cellulosic raw materials of good quality has become limited due to a world-wide shortage of natural resources and pollution problems with pulp mills and the cellulose derivatives industry has been obliged to change its raw materials from cotton linters to wood pulp and from high grade wood pulp to low grade wood pulp. Cellulose acetate produced from low grade wood pulp as a raw material especially tends to decreased commercial value because of its yellowness.
The origin of the yellowness of cellulose acetate products has been discussed in many publications and various theories have been presented. The most widely accepted theory is that non-cellulosic substances such as xylan in a pulp are the source of yellowness. (J. D. Wilson, R. S. Tabke, Tappi, 57, 77 (1974), F. L. Wells, W. C. Shattner, A. Walker, Tappi, 46, 581 (1963))
We have observed that the yellowness of cellulose acetate flake decreases with a reduction in the hemicellulose content (e.g., xylan) when the flake is washed with a lower aliphatic alcohol (e.g., methanol) to yield a dark coloured washing solvent (as described in Japanese Patent Publications Nos. 48-23542 and 48-23543). It can thus be understood that the extraction of cellulose acetate in flake form with a lower aliphatic alcohol can improve the whiteness of the flake. However, this washing procedure cannot be applied commercially because it leads to additional expense in the production cost of cellulose acetate.